Java 8 autoboxing + generics: different behaviour with variable vs. method

Question

I found a piece of code that after switching from Java 7 to Java 8 stopped compiling. It does not feature any of the new Java 8 stuff like lambda or streams.

I narrowed

Answer 1

CompileMe.java:4: error: incompatible types: bad type in

conditional expression

      Double d = g == null ? 0 : g.getData(); // type error!!!

int cannot be converted to Double

Here 0 is integer and you are putting it inside Double.

Try this

public class CompileMe {
    public static void main(String[] args) {
        GenericData g = new GenericData(1d);
        Double d = g == null ? 0d : g.getData(); // type error!!!
        Double d2 = g == null ? 0d : g.data; // now why does this work...
        System.out.println(d);
        System.out.println(d2);
    }
}

class GenericData {
    public T data;
    public GenericData(T data) {
        this.data = data;
    }
    public T getData() {
       return data;
    }
}

Or use double literal instead of Double wrapper class

    public class CompileMe {
    public static void main(String[] args) {
        GenericData g = new GenericData(1d);
        double d = g == null ? 0 : g.getData(); // type error!!!
        double d2 = g == null ? 0 : g.data; // now why does this work...
        System.out.println(d);
        System.out.println(d2);
    }
}

class GenericData {
    public T data;
    public GenericData(T data) {
        this.data = data;
    }
    public T getData() {
        return data;
    }
}

Because widening and boxing will not take place simultaneously.